The inventive concept relates to photolithography. More particularly, the inventive concept relates to photomasks and to methods of forming photomasks.
The fabricating of semiconductor devices entails the forming of fine patterns on a semiconductor substrate using lithographic techniques in which light is projected through a photomask and onto a photosensitive layer on a substrate. The photomask has a pattern corresponding to that to be formed on the substrate. Thus, the photosensitive layer on the substrate is exposed to an image of the photomask pattern. After the exposure process, the photosensitive layer is developed to “pattern” the layer. The patterned photosensitive layer may then be used as a mask during an etching process in which that portion of the substrate or layer exposed by the photosensitive pattern is removed.
For semiconductor devices to evolve and offer greater performance on a smaller scale, finer and finer patterns must be formed. To this end, light of increasingly shorter wavelengths is being used in lithography. For example, the most common light sources once used in lithography processes generated light whose wavelength was either that of the G-line (436 nm) or I-line (365 nm). However, recently, lithography processes are being carried out using light of a wavelength in the deep ultraviolet or extreme ultraviolet (EUV) band.
Meanwhile, EUV lithography processes are generally performed using a reflective optical system, as opposed to a refractive optical system, because the light of the EUV band is absorbed by most refractive optical materials. Thus, research into various methods of controlling the reflectance of the EUV photomask has been conducted with an aim towards reducing the critical dimension, such as line width, of the pattern that can be formed on a substrate by EUV lithography.